Resolves: fdo#72174 convert_add from "at" or "atm" to "Pa" gives same answer

Change-Id: I76c1a0bd7cc93cf60ffa8da08200969a7315a8c3
Reviewed-on: https://gerrit.libreoffice.org/6869
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>